1. Field of the Invention
This invention relates generally to heat exchange systems, and more particularly to a method and device for controlling the temperature of molds. Even more particularly, the invention relates to a device and method for rapidly heating and cooling a mold during a molding process.
2. Description of the Background Art
In the manufacturing industry, injection molding is a common process for producing parts. Conventional injection molding requires the use of a constant-temperature mold, an injection molding machine, and raw plastic material. The mold includes a core side and a cavity side which, together, define the shape of a part or multiple parts being molded. The injection molding machine is responsible for heating the raw plastic material until liquefied, then injecting the molten plastic into the mold.
The conventional injection molding process includes a clamping stage, an injection stage, a cooling stage, and a removal stage. In the clamping stage, the core side and cavity side are clamped shut while the raw plastic is being melted within the injection mold machine. Then, in the injection stage, the molten plastic is injected and packed into the cavity of the mold. In the cooling stage, the mold remains clamped while the molten plastic cools and solidifies into the final part. Finally, in the removal stage, the mold is unclamped and the newly formed part is removed via, for example, an ejector pin. After the part is ejected, the cycle is typically repeated.
There are several well known problems that limit the use of constant-temperature injection molding. For example, the surface finish and structural integrity of the final part is substantially compromised when a known problem called “premature freezing” occurs. Premature freezing occurs during the injection stage when the temperature of the mold is less than the temperature of the molten plastic. As the molten plastic contacts the cooler interior wall of the mold, freezing of the plastic begins at the plastic-to-mold interface before the entire shot of molten plastic is injected and packed into the mold. This is particularly undesirable when molding long and thin parts, optical parts, and parts that require a glossy surface finish. Further, uneven solidification is another known problem that can result from premature freezing or when the molten plastic is cooled too slowly. If the molten part is cooled too slowly, certain areas of the molten plastic freeze and, therefore, shrink before other areas. Of course, if this occurs, the final part will likely include several defects (e.g., dimensional distortion, knit lines, poor surface finish, etc.).
It is known that these problems can be avoided by controlling the temperature of the mold itself during the molding process. In particular, premature freezing is avoided by heating the mold above the plastics heat distortion temperature (HDT) before the molten plastic is injected. Once the molten plastic is injected into the hot mold and allowed to set, the mold is cooled to minimize uneven solidification. Heating the mold is typically done by pumping hot fluid through a tunnel, or network of tunnels, formed in the mold. Likewise, cooling is done by pumping cool fluid through a separate tunnel, or network of tunnels, formed in the mold.
Current molding facilities typically employ a remote boiler system and a remote cooling reservoir, both of which supply fluid to multiple molds located throughout the facility. The molds are connected to the boiler via hot fluid supply lines that carry hot fluid to each of the molds. The molds are also connected to the cooling reservoir via cool fluid supply lines that carry cool fluid to each of the molds. After fluid is passed through the molds, it can be carried back to either the boiler or the cooling reservoir through fluid return lines. Fluid carried back to the boiler is reheated and used again while fluid carried back to the cooling reservoir is cooled and used again.
Although current molding facilities can control the temperature of multiple molds, there are several disadvantages. For example, the initial cost and development of such facilities is very high. As another example, daily production costs in current facilities are also high. Much of these production costs are a result of high daily energy and maintenance costs. Daily energy costs are high primarily because boiler systems and cooling systems consume a great deal of energy during operation. Additionally, thermal energy dissipates through the fluid lines and is, therefore, lost as fluid is moved long distances to and from the molds. Maintenance costs are high as a result of preventative maintenance that is required to prevent downtime of the boiler system and/or cooling system during production because downtime of the boiler system and/or cooling system likely results in downtime of the entire facility. As another example, it is difficult to move or relocate molding equipment throughout such facilities because heating and cooling of the molds can only be done in locations that provide access to the fluid supply and return lines.
What is needed, therefore, is a mold-temperature control system that is less expensive. What is also needed is a more energy-efficient mold-temperature system that requires less preventative maintenance during operation. What is also needed is a mold-temperature control system that can operate without being connected to a remote fluid source.